The present invention relates to the production of prepregs and resin varnishes useful for the production of printed wiring boards including multilayer printed wiring boards. The present invention also relates to resin compositions useful for the production of printed wiring boards. The present invention further relates to laminates for printed wiring boards which are produced by using the prepregs or the resin varnishes.
As electronic instruments have been miniaturized and improved in performance, the mounting density of laminates for printed wiring boards has been increased by making the laminates thinner and more multilayered and by reducing the diameter and hole pitch of through holes. Thus, the requirements of laminates for heat resistance, drilling processability and insulation properties are becoming more strict.
Laminates are generally produced by impregnating a base material with a resin varnish and drying to form a prepreg, and superposing a desired number of sheets of the prepreg on each other with a metal foil superposed on one or both sides, and heating and pressing with a parallel heat machine. Multilayer printed wiring boards are generally produced by superposing prepregs on each side of an inner printed wiring board produced by circuit-patterning a double-sided metal-clad laminate, and then superposing metal foil on the prepregs, and then bonding the superposed composite with heat and pressure between a parallel heat machine.
To improve the heat resistance and insulation properties, the improvements in the properties of the cured resins in the laminates have widely been tried, for example, by increasing the Tg (glass transition temperature) of the resins. Such improvements in resin properties, however, became insufficient to satisfy the required properties.
In parallel with the improvements in the resin properties, a long-standing investigation has been made to increase the base material/resin interfacial adhesion. Interface control is a very important technique since the degree of interfacial adhesion directly influences the resistance against heat and moisture, drilling processability, insulation properties and electrolytic corrosion resistance of laminates.
Another means is the addition of inorganic fillers. Inorganic fillers are used not only as extenders but also for improving dimensional stability and resistance against moisture and heat, and selective use of specific fillers has recently been investigated to attain excellent properties, such as high dielectric constant, efficient radiation and high strength. However, fillers added to resin varnishes precipitate slowly and should be dispersed again by stirring or the like at the time of coating. When the precipitation is considerable, the precipitated fillers cohere on the bottom of containers, and become difficult to disperse by stirring alone. During the production of prepregs, fillers also precipitate in apparatuses where varnishes stay, for example, in varnish tanks and impregnating tanks, and gradually adhere to rolls, etc. This deteriorates the appearance of prepregs considerably. In laminates, the inhomogeneously dispersed fillers decrease the interfacial adhesion between base materials and resins or between the fillers and resins, thereby deteriorating the drilling processability and insulation properties.
A general method of improving the base material/resin interfacial adhesion is the pretreatment of base materials, such as glass woven fabric, with surface treating agents, such as coupling agents. Prepregs are produced by impregnating a surface-treated base material with a resin varnish and then drying to semi-cure the resin. During the drying step, the reaction of the surface treating agent on the surface of the base material with the resin proceeds to some degree, and further proceeds during the following heating step for forming laminates or multilayer printed wiring boards, to increase the adhesion between the base material and the resin. A known method of further increasing the adhesion is to improve the reactivity of surface treating agents with resins by varying the number and kinds of the organic functional groups on the conventional surface treating agents, such as silane coupling agents (Japanese Patent Application Unexamined Publication No. 63-230729 and Japanese Patent Application Examined Publication No. 62-40368). The improved reactivity with resins, however, merely gives a rigid and thin layer on the interfaces, and cannot decrease the residual stress set up on the interfaces and cannot improve the adhesion remarkably.
Another method of improvements, including a reduction of the residual stress on interfaces, is the use of surface treating agents together with long chain polysiloxanes which reduce such stress (Japanese Patent Application Unexamined Publication Nos. 3-62845 and 3-287869). The method, however, is far from effective in increasing the interfacial adhesion since the reactivity of surface treating agents with long chain polysiloxanes is very poor under usual treating conditions, long chain polysiloxane have no alkoxyl groups reactive to base materials, and the hydrophobic groups, such as methyl groups, on long chain polysiloxanes disturb the impregnation of base materials with the long chain polysiloxanes.
It has also been tried to improve the dispersibility of fillers by using fillers treated with surface treating agents, such as coupling agents. However, commercially available surface-treated fillers are expensive, and the kinds thereof are too little to select a proper filler for each of various resin blends. For further improvements in functions, the amount of fillers blended in resins is now increasing. As the amount of fillers increases, the above-described precipitation and adhesion to rolls become more considerable, requiring further improvements in dispersibility and thixotropy. The conventional treatments with coupling agents cannot satisfy such requirements.
In addition, the treatments of fillers with surface treating agents are generally performed by dipping or spraying using diluted solutions of the surface treating agents, followed by drying with heat. The drying step raises two problems, namely, the formation of physically adsorbed layer of oligomerized coupling agents on the filler surfaces, and the cohesion of fillers, which necessitates grinding at the time of blending with resin varnishes. Such grinding roughens the surface treating agent layers on fillers. The physically adsorbed layers and the uneven layers of surface treating agents decrease the interfacial adhesion in laminates.
It is also proposed in Japanese Patent Application Unexamined Publication No. 61-272243 to add coupling agents directly into resin varnishes under preparation. The layers formed from commercial coupling agents are also too rigid and thin to improve the interfacial adhesion between base material/resin. On the other hand, this method somewhat prevents the cohesion of fillers because of the high viscosity of the resin varnishes containing resins. From the viewpoint of the filler/resin interfacial adhesion, the coupling agents cannot align selectively and uniformly on the filler surfaces, and cannot bond the fillers and resins sufficiently.
FIG. 1 shows a schematic view illustrating an ideal state of the surfaces of base materials or fillers which are treated with conventional silane-coupling agents. Chemically adsorbed silicone chains 2 (silicone chains adsorbed via chemical bonding with base materials or fillers) form a layer of a certain thickness on the surface of a base material or inorganic filler 1, and improve the adhesion to a resin layer 4. The layer of chemically adsorbed silicone chains 2 bears physically adsorbed silicone chains 3 (silicone chains having no chemical bonds with the base material or filler) thereon. However, it is commonly recognized that the state as shown in FIG. 2 is the actual state made by industrial surface treatments of base materials or fillers, which are performed in a short time. That is, even the chemically adsorbed silicone chains 2 do not cover the surface of the base material or filler 1 uniformly, and there are many physically adsorbed silicone chains 3, which are easily eluted into the resin layer 4. Such a defective chemically adsorbed layer cannot fully exhibit its bonding function. The physically adsorbed layer may inhomogenize and weaken the resin which is cured near the interface, and may rather decrease the adhesion.
In Japanese Patent Application Unexamined Publication No. 1-204953 is proposed to solve such problems by treating inorganic fillers with a linear polysiloxane which has both trialkoxyl groups reactive to inorganic fillers and organic functional groups reactive to resins. As shown in FIG. 3, chemically adsorbed linearly long polysiloxane chains 6 tend to lie on the filler surface due to the alignment of hydrophobic groups, such as methyl groups, and hardly penetrate into the resin layer 4. Also such long chains bond to the inorganic filler surface at several sites per molecule and tend to form a rigid layer. Even if penetrated into the resin, the siloxane chains are surrounded by the resin, and the decrease in the interfacial stress does not correspond to the chain length. Further, the physically adsorbed long polysiloxane chains 7 easily form large cyclic chains 5, which tend to deteriorate the properties of cured resins.
The object of the present invention is to solve the above-described problems in the prior arts by providing novel means for improving the interfacial adhesion between base materials or inorganic fillers with resins, thereby enabling the production of laminates and multilayer printed wiring boards which are excellent in drilling processability and insulation properties.
The inventors found that the above-described problems can be solved by using, as surface treating agents for base materials and inorganic fillers, a silicone oligomer having functional groups reactive to the hydroxyl groups which are present structurally or by moisture absorption on the surfaces of base materials and fillers, or by using novel method for producing resin varnishes containing inorganic fillers treated with such a silicone oligomer, and have consequently completed the present invention.
That is, the present invention provides a method of producing a prepreg for printed wiring boards, comprising treating a base material with a silicone oligomer, impregnating the treated base material with a resin varnish, and drying the impregnated base material, the silicone oligomer containing at least one kind of siloxane units selected from the group consisting of trifunctional siloxane units (RSiO3/2) and tetrafunctional siloxane units (SiO4/2), having a polymerization degree of 2 to 70 and having at least one functional end group reactive to a hydroxyl group, each R being an organic group, and the R groups in the silicone oligomer being identical with or different from one another.
The present invention also provides a laminate for printed wiring boards (hereinafter, it may be called xe2x80x9claminate (a)xe2x80x9d), which is produced by superposing two or more sheets of the above-described prepreg, with a metal foil superposed on one or both sides of the superposed sheets of the prepreg, to form a superposed composite, and then bonding the superposed composite with heat and pressure.
The present invention further provides a method of producing a resin varnish for printed wiring boards comprising dipping an inorganic filler in a treating liquid for surface treatment, and then compounding a resin material directly with the treating liquid containing the treated inorganic filler, wherein the treating liquid comprises a silicone oligomer dissolved in a solvent, and the silicone oligomer contains at least one kind of siloxane units selected from trifunctional siloxane units (RSiO3/2) and tetrafunctional siloxane units (SiO4/2), wherein each R is an organic group and the organic groups R in the silicone oligomer are identical with or different from one another, has a polymerization degree of 2 to 70, and has at least one functional end group reactive to a hydroxyl group.
The present invention also provides a laminate for printed wiring boards (hereinafter, it may be called xe2x80x9claminate (b)xe2x80x9d), which is produced by impregnating a base material with the resin varnish produced by the above described method, drying the impregnated base material to form a prepreg, superposing two or more sheets of the prepreg, with a metal foil superposed on one or both sides of the superposed sheets of the prepreg, to form a superposed composite, and then bonding the superposed composite with heat and pressure.
The present invention further provides a resin composition for printed wiring boards (hereinafter, it may be called xe2x80x9cresin composition for printed wiring boards (A)xe2x80x9d or xe2x80x9cresin composition (A)xe2x80x9d) comprising a resin material and the above-described silicone oligomer.
The present invention also provides a laminate for printed wiring boards (hereinafter, it may be called xe2x80x9claminate (c)xe2x80x9d), which is produced by impregnating a base material with the resin composition (A), drying the impregnated base material to form a prepreg, superposing two or more sheets of the prepreg, with a metal foil superposed on one or both sides of the superposed sheets of the prepreg, to form a superposed composite, and then bonding the superposed composite with heat and pressure.
The present invention further provides a resin composition for printed wiring boards (hereinafter, it may be called xe2x80x9cresin composition for printed wiring boards (B)xe2x80x9d or xe2x80x9cresin composition (B)xe2x80x9d) comprising a resin material and an inorganic filler treated with a silicone oligomer which contains at least one kind of siloxane units selected from trifunctional siloxane units (RSiO3/2) and tetrafunctional siloxane units (SiO4/2), wherein each R is an organic group and the organic groups R in the silicone oligomer are identical with or different from one another, has a polymerization degree of 2 to 70, and has at least one functional end group reactive to a hydroxyl group.
The present invention also provides a laminate for printed wiring boards (hereinafter, it may be called xe2x80x9claminate (d)xe2x80x9d), which is produced by impregnating a base material with the resin composition (B), drying the impregnated base material to form a prepreg, superposing two or more sheets of the prepreg, with a metal foil superposed on one or both sides of the superposed sheets of the prepreg, to form a superposed composite, and then bonding the superposed composite with heat and pressure.